1. Field of the Invention
This invention relates to nonlinear optical devices, and more particularly to nonlinear optical devices incorporating nonlinear optical materials formed from alternate layers of charge carrier and charge barrier materials which form multiple quantum well (MQW) structures, and incorporating low diode lasers as a light source.
2. Description of the Prior Art
Heretofore at room temperature nonlinear optical materials have required more power than is available from a milliwatt range solid state laser in order to saturate their absorption and thereby cause their index of refraction to vary with incident light intensity.
Nonlinear effects of light intensity on the index of refraction may be represented by the expression EQU n=n.sub.L +n.sub.2 I
where n is the index of refraction of the material, n.sub.L is the low intensity index of refraction, I is the intensity of the incident light beam in, for example, units of Watts/cm.sup.2, and n.sub.2, is the coefficient of nonlinearity of the material and may be expressed in the units cm.sup.2 /Watt.
Measurements of n.sub.2 at a wavelength of 1.06 microns for a number of materials were reported by Moran et al. in the article "Interferometric Measurements of the Nonlinear Refractive Index Coefficient Relative to CS.sub.2 in Laser System Related Materials", in the IEEE Journal of Quantum Electronics, Vol. OE-11, June 1975, p. 259, and showed that n.sub.2 for the material CS.sub.2 is from 10 to 100 times larger than for a variety of materials used in laser construction. Moran et al. give n.sub.2 =3.10.sup.-14 cm.sup.2 /Watt for CS.sub.2. A measurement of the third order nonlinear susceptibility of silicon at a wavelength of 1.06 microns was reported by Jain et al. in the article "Degenerate Four-Wave Mixing Near the Bandgap of Semiconductors" in Applied Physics Letters, Vol. 35, September 1979, p. 454, as 8.10.sup.-8 esu, and this value is equivalent to a value of n.sub.2 =3.5.times.10.sup.-10 cm.sup.2 /Watt. The above values of n.sub.2 are too small to make the above materials useful in a nonlinear optical device in which a milliwatt diode laser is used as the light source.
An optical bistable Fabry-Perot cavity containing a saturable absorber is described by A. Szoke, in U.S. Pat. No. 3,813,605 issued to A. Szoke on May 28, 1974. Szoke suggests using a CO.sub.2 laser at a wavelength of 10.6 microns with SF.sub.6 gas as the saturable absorber, and also suggests using a CW He-Ne laser but does not suggest a saturable absorber for the He-Ne laser. Szoke further suggests using the Fabry-Perot cavity and nonlinear absorber to produce short time duration optical pulses, as an optical amplifier inverter, as an optical Schmidt trigger, and as an optical flip-flop. However, the devices taught by Szoke use absorption saturation and not nonlinear refractive index changes.
A bistable optical interface was described by P. W. Smith et al. in the article "Optical Bistability at a Nonlinear Interface" in Applied Physics Letters, Vol. 35, December 1979 at p. 846. There, a glass to CS.sub.2 interface was employed, and an incident intensity of 7.5.times.10.sup.9 Watt/cm.sup.2 was required to destroy the total reflection. Again, this light intensity is too great to make the bistable interface usable with a milliwatt diode laser.
Gibbs et al. in the article "Optical Bistability in Semiconductors" in Appl. Phys. Letters, Vol. 35, September 1979, at p. 451, observed optical bistability in a device comprising a 4.1 micron-thick layer of GaAs sandwiched between two 0.21 micron layers of Ga.sub.0.58 Al.sub.0.42 As layers in a temperature range of 5.degree. to 120.degree. K. The exciton peak was at approximately 818 nanometers. Bistability occurred at incident optical intensity of approximately 1.0 to 1.5 milliwatts/square micron, but above 120.degree. K. the bistability disappeared.
Gibbs et al. in the article "Room Temperature Excitonic Optical Bistability in a GaAs-GaAlAs Superlative Etalon" in Appl. Phys. Letters, Vol. 41, August 1982, at p. 221, observed optical bistability in a multiple quantum well device used in a bistable Fabry-Perot cavity, and these authors disclosed the above-mentioned bistable Fabry-Perot cavity using a MQW in the Optical Sciences Center Oscillations, published by the Optical Sciences Center of the University of Arizona, No. 229, on Mar. 26, 1982. The MQW device taught by Gibbs et al. comprised 61 periods; each period containing a 336 Angstrom GaAs layer and a 401 Angstrom layer of Ga.sub.0.73 Al.sub.0.27 As, at a temperature of 300.degree. K. Dielectric coatings were deposited on both surfaces to increase reflectivity to nearly 90 percent between 820 to 890 nanometers. Bistability was observed at 300.degree. K. at a wavelength of 881 nanometers and at an input light intensity between approximately 70 milliwatts and 100 milliwatts focused to a spot size of 5-10 micron diameter. Again, the light intensity used by Gibbs et al. exceeds the intensity which can be conveniently supplied by a milliwatt power diode laser.